1. Field of the Invention
The present invention relates to an imaging apparatus for photographing an infrared composite image that is a combination of a visible-light image and an infrared image, and an image signal processing device for generating an image signal that displays the infrared composite image.
2. Description of the Related Art
Fluorescent materials are used in a technique for detecting the structure of a physical object that cannot be clearly perceived in a visible-light image, or detecting the presence of a specific substance in a physical object. For example, in a microscope disclosed in JP-A 10-325798, an antibody combined with the fluorescent material indocyanine green (below referred to as ICG) is used as a probe, and the presence of microcarcinoma in a biological tissue section is detected by observing the fluorescence emitted by the probe that has the property of tending to accumulate in cancerous tissue.
ICG is a pigment that absorbs near-infrared light of about 800 nm and emits approximately near-infrared fluorescent light of about 840 nm. The pigment can be administered as a contrast agent, and infrared images observed thereby can be used in the diagnosis and treatment of diseases. For example, an image of a blood vessel can be observed by injecting ICG into the blood vessel. ICG can also be used to examine liver function by using the property of ICG to combine with the proteins in the blood and to be selectively taken up by the liver.
In surgical operations it is necessary that doctors be able to visually identify the object region. In such situations, there are cases in which it is convenient to obtain images that make it possible to identify the position of a blood vessel present on below the surface of the object region in addition to the usual images obtained as visible-light images. From this perspective, images (infrared composite images) in which an infrared image obtained through a fluorescent contrast agent is displayed in combination with a visible-light image are desirable.
An imaging apparatus that can create visible-light images and infrared images is necessary to generate an infrared composite image. CCD image sensors and other solid-state image sensors have sensitivity not only to visible light but also to near-infrared light. Therefore, it has been proposed to create infrared composite images by using such solid-state image sensors.
The sensitivity of a solid-state image sensor to infrared light is the source of image degradation for the usual goal of creating visible-light images. For example, a problem of a solid-state image sensor provided with color filters and used to create color images is that correct color representation cannot be achieved with incident light that includes infrared-light components because each light-receiving pixel for detecting light component rays that correspond to RGB and other colors also has sensitivity to infrared light. In order to solve this problem, a configuration has been proposed for a solid-state image sensor wherein components having selective sensitivity to infrared light are mixed in an array of light-receiving pixels, as indicated below in JP-A 2006-237737. The light-receiving pixels having sensitivity to infrared light (infrared light-receiving pixels) in the solid-state image sensor are provided in order to eliminate the effect of infrared light on the light-receiving pixels having sensitivity to visible light (visible light-receiving pixels). In other words, a signal obtained from infrared light-receiving pixels is used as a reference signal for estimating and removing the infrared light component contained in the signal from the visible light-receiving pixels.
Infrared light emitted by a fluorescent material is generally of low intensity. Therefore, a problem is encountered in an application in which such a fluorescent contrast agent is administered and the area to be operated on is photographed; i.e., only a blurred infrared composite image can be obtained using a conventional, general solid-state image sensor composed of visible light-receiving pixels.